In the implantable medical device field, a medical device, configured to perform a desired medical function, is implanted in the living tissue of a patient so that a desired function may be carried out as needed for the benefit of the patient. Numerous examples of implantable medical devices are known in the art, ranging from implantable pacemakers, cochlear stimulators, muscle stimulators, glucose sensors, and the like.
Many implantable medical devices are configured to perform only the stimulation function, i.e., to stimulate on command a prescribed muscle tissue in order cause the muscle to contract. An example of a tiny implantable stimulator is shown, e.g., in U.S. Pat. Nos. 5,324,316 or 5,358,514.
Other implantable medical devices are configured to perform only the sensing function, i.e., to sense a particular parameter, e.g., the amount of a specified substance in the blood or tissue of the patient, and to generate an electrical signal indicative of the quantity or concentration level of the substance sensed. Such electrical signal is then coupled to a suitable controller, which may or may not be implantable, and the controller responds to the sensed information in a way to enable the medical device to perform its intended function, e.g., to display and/or record the measurement of the sensed substance. An example of an implantable medical device that performs the sensing function is shown, e.g., in U.S. Pat. No. 4,671,288.
Still other implantable medical devices are configured to perform both the sensing and stimulating function. In such instances, the medical device typically includes separate sensing, stimulating and control circuits. The sensing circuit senses the presence or absence of a particular parameter or substance. The control circuit analyzes the information sensed by the sensor and determines whether a stimulation current pulse is needed. If a stimulation current pulse is needed, the control circuit directs the stimulating circuit to provide a specified stimulation current pulse. A pacemaker is a classic example of an implantable medical device that performs both the sensing function (sensing whether the heart needs to be stimulated and at what rate) and the stimulating function (stimulating the heart as needed to maintain a desired heart rhythm).
As medical devices have become more sophisticated, there is a continual need to use more than one sensor. For example, in some instances, more than one sensor is needed to measure more than one substance or physiological parameter. In other instances, more than one sensor may be needed to measure or sense the same substance or physiological parameter at different locations within the patient's body. Similarly, depending upon the medical application involved, there may be a need to stimulate muscle tissue at more than one location in the body. One way of providing stimulation at multiple locations is to implant separate stimulators at each desired location and then to coordinate the operation of the stimulators so as to provide a desired result. See, e.g., U.S. Pat. No. 5,571,148.
Whenever multiple sensors and/or multiple stimulators are implanted and are intended to be used in concert to achieve a desired medical function, there is a need to connect or couple such separate multiple sensors/stimulators to a single control circuit or common control point. Sometimes the control function is performed external to the patient, in which case the sensors/stimulators are connected to an implanted telemetry circuit or equivalent; or, alternatively, a telemetry circuit is included as part of each sensor that allows data and commands to be sent, transferred, or otherwise coupled across the tissue/skin of the patient between an external control device and the implanted sensor/stimulator. At other times, the control function is performed by an implantable control circuit, usually connected directly to the implanted sensors/stimulators. When an implanted control circuit is used, it usually includes a telemetry circuit, or equivalent circuit, that allows the implanted control circuit to communicate with an external programmer, thereby allowing the implanted control circuit to be programmed, or otherwise modified and/or monitored, by the external programmer.
When multiple sensors/stimulators are used, several problems must be addressed. For example, unless each of the multiple sensors/stimulators are connected to a common controller and/or telemetry circuit, each sensor/stimulator must employ its own telemetry circuit, or equivalent circuit, that allows it to be monitored and/or controlled. Such individual telemetry or communication circuits may add undue complexity to the implanted sensors/stimulators, increasing the size, weight and/or power consumption of the sensors. What is needed are relatively simple sensors and stimulators that may be implanted at multiple locations within the patient, yet operate independent of each other in an efficient and effective manner.
When multiple sensors/stimulators are directly monitored and/or controlled by a control circuit, there must be a direct connection, i.e., at least a separate conductor and a return path, for each sensor/stimulator. If the number of sensors/stimulators is large, the number of separate conductors that are required to control and/or monitor such sensors/stimulators can become unwieldy. The number of conductors can become especially large and difficult to manage when each sensor requires more than two conductors, e.g., as when each sensor performs multiple functions, requiring a separate output conductor for each function, in addition to conductors to carry power to the sensor. Moreover, the output signal from many sensors, i.e., the signal that provides a measure of the parameter or substance being monitored or sensed, is typically a very low level analog signal that cannot be transmitted over very long distances without amplification or buffering. That is, such low level signals are easily corrupted with noise, particularly when the conductors are placed in a very hostile environment (e.g., within living tissue, which is equivalent to being immersed in salt water). Low level signals in a hostile environment result in a signal-to-noise (S/N) ratio that is unacceptably low. A unacceptably low S/N ratio, in turn, dictates that signal amplification and/or special buffering circuits be employed. Such signal amplification and/or buffering, however, disadvantageously require additional circuitry, thereby increasing the complexity, size and weight of the device, and further require additional operating power. What is clearly needed, therefore, are sensors/stimulators that can be readily operated in a multiple sensor/stimulator configuration, yet require a minimum number of conductors to connect the sensors/stimulators to a control circuit, and wherein a high S/N ratio can be maintained for data and command signals that are transmitted to and from the sensors/stimulators.